1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium (e.g., a transfer sheet) according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Market demand for high-speed image forming apparatuses requires that a toner image be fixed on a recording medium properly in the fixing device even when the image forming apparatus forms the toner image on the recording medium at high speed with a shortened warm-up time and first print.
To address such demand, the fixing device may include a heating member such as a heat-conductive metal pipe provided inside a loop formed by an endless belt and facing an inner circumferential surface of the belt. A heater provided inside the heating member heats the heating member and the heating member heats the whole belt.
More specifically, the heating member is pressed against a pressing rotary member located outside the loop formed by the belt via the belt to form a nip portion between the pressing rotary member and the belt that nips a recording medium bearing a toner image as the recording medium passes through the nip portion. A reinforcement member is provided inside the heating member to press against the pressing rotary member via the heating member and the belt so as to reinforce the heating member at the nip portion. The heater provided inside the heating member heats the belt via the heating member.
With such a structure, the recording medium bearing the toner image passing through the nip portion receives heat from the belt and pressure from the pressing rotary member to fix the toner image on the recording medium.
However, in such a fixing device, sufficient time is needed to warm up the belt. Otherwise, the belt may not be heated uniformly in a circumferential direction of the belt. Uneven or incomplete heating of the belt in the circumferential direction may cause the toner image to be fixed on the recording medium unevenly or may cause localized hot offsets on the toner image.
One prominent reason why the heater may not heat the heating member uniformly in the circumferential direction of the heating member may rest with the structure of the heating member itself. That is, the heating member may be constituted so as to include a primary heating portion directly heated by the heater and a secondary heating portion continuous with and adjacent to the primary heating portion and heated indirectly by heat conducted from the primary heating portion.
Also, the reinforcement member may block radiation heat generated by the heater toward the heating member. Accordingly, a part of the heating member may not be heated by the radiation heat.
Failure of the heater to heat the heating member uniformly in the circumferential direction of the heating member may result in failure of the heating member to expand thermally uniformly in the circumferential direction of the heating member. Consequently, parts of the heating member may come into substantial frictional contact with the belt, interfering with movement of the belt and adversely affecting the durability of the belt.